1. Field of the Invention
The present invention relates to a pre-transfer system of an image forming apparatus, and, more particularly, to a pre-transfer system of an image forming apparatus capable of enhancing the pre-transfer efficiency by placing a pre-transfer lamp ("PTL") upstream of the delivery path of print papers to cause the light emitted from the PTL to be scanned directly onto a photosensitive drum.
2. Description of the Related Art
In general, an image forming apparatus employing an electrophotographic developing technique such as a facsimile, printer or complex machine incorporating multiple features, includes a pick-up roller, an electrification roller, a photosensitive drum, a transfer roller, a developing roller, a supply roller, a fixer and a laser scan unit ("LSU ") for printing certain print data onto a print paper.
In the printing process, the electrification roller electrifies uniformly the photosensitive substance coated on the external surface of the photosensitive drum while rotating, and the light generated from the LSU forms an electrostatic latent image to be printed on the electrified photosensitive drum. Then, a voltage difference is generated between the supply roller to which a higher supply voltage is applied and the developing roller to which a lower voltage is applied. Therefore, negative charges move from the supply roller to the developing roller. In this way, toner supplied to the developing roller is coated on the electrostatic latent image formed on the surface of the photosensitive drum to form a visible image. The high voltage of the transfer roller transfers the visible image formed by the toner coated the surface of the photosensitive drum to the delivered recording paper. The visible image transferred to the recording paper is fixed on the recording paper by the high temperature and high pressure of a heating roller and a pressing roller provided in the fixer, thereby completing the printing process.
At this time, the above-described supply, developing, transfer and electrification voltages are continuously applied to the supply roller, the developing roller, the transfer roller and the electrification roller, respectively, until the printing process is completed. In addition, the heating roller in the fixer is maintained in the turned-on state until the printing process is completed.
On the other hand, the PTL is installed between the developing roller and the transfer roller. Light of 650 nm wavelength generated at the PTL is scanned onto the surface of the photosensitive drum to lower the potential of the toner coated on the photosensitive drum, thereby decreasing the engagement force of the toner to the photosensitive drum and enhancing the transfer efficiency.
That is, the conventional PTL is located downstream of the paper delivering path in the image forming apparatus, and the potential of the toner coated on the surface of the photosensitive drum is increased to a value near the ground level by the light emitted to the toner prior to the pre-transfer operation, thereby decreasing the engagement force of the toner and increasing the transfer efficiency. At this time, the light emitted from a light emitting diode ("LED") in the PTL is focused by a convex lens in a dot pattern on the surface of the photosensitive drum.
In such a conventional pre-transfer system of the image forming apparatus, however, the following problems arise. When the PTL is activated in order to increase the potential of the toner to a value near the ground level at the downstream of the recording paper, since the light emitted from the PTL passes through a separate medium, that is, the printing paper, before it is scanned to a focus position with respect to the photosensitive drum set initially, the light may be refracted or spread due to the medium, thereby resulting in a decrease of the light intensity or a dispersion of light due to the medium. In this case, the intensity of light is reduced. Particularly, since the light emitted from the PTL must pass through the medium, the light intensity decreases and the transfer efficiency is very poor.
Further, since the transmission rate and the light collecting degree are affected by the properties, e.g., thickness, resistance and texture pattern, of the recording paper, the amount of light needs to be varied depending on the kind of recording paper. Therefore, since the kind of recording paper should be identified and the light amount should be controlled on a case by case basis according to the kind of the paper identified, additional sensors and a high performance central processing unit are required.
In addition, since the PTL is placed downstream of the delivering path of the recording paper, the PTL should be located adjacent to the photosensitive drum so as to enhance the transmission rate of the light. In this case, since the scanning width of the light emitted from the light emitting diode in the PTL is narrow, a plurality of light emitting diodes is required, thereby resulting in a complicated structure of the pre-transfer system.
Moreover, since the light emitted from the PTL is transmitted to the paper and then reaches the surface of the photosensitive drum, the lens for collecting the light emitted from the PTL is a convex lens in order to provide a sufficient amount of the light to the photosensitive drum. Thus, the area which the light emitted from PTL reaches becomes very small. In such a focusing method, since the area reached by the light is very small, the number of light emitting diodes that must installed in the pre-transfer system is increased.